More U.S. women are using marijuana during pregnancy than in previous years to treat nausea and morning sickness, a new study suggests.

The data presents potentially serious medical concerns because of previous studies indicating that infants who were exposed to marijuana are more likely to be anemic, have lower birth weight, and be placed in neonatal intensive care than infants of mothers who did not use marijuana.

According to the study published Monday in the Journal of the American Medical Association, nearly four per cent of pregnant women between the ages of 18 and 44 reported in 2014 they had used marijuana in the past month, compared with 2.4 per cent in 2002.

“Some sources on the Internet are touting marijuana as a solution for the nausea that commonly accompanies pregnancy,” said Nora D. Volkow, director of the National Institute on Drug Abuse, writing in an editorial published online with the study. However, she said, “doctors must be aware of the risks involved and err on the side of caution by not recommending the drug for pregnant patients.”

Some sources on the Internet are touting marijuana as a solution for the nausea that commonly accompanies pregnancy

Some data suggests pregnant women are turning to marijuana specifically during the first trimester of pregnancy, the period that presents the greatest risks of drug exposure to the fetus, Volkow wrote.

Although the influence of marijuana on human brain development is not entirely clear, studies have shown links between prenatal marijuana exposure and impaired functions such as impulse control, visual memory, and attention during school years. An ongoing study also found an association between prenatal marijuana exposure and restricted fetal growth during pregnancy, as well as increased frontal cortical thickness among school-aged children.

The American College of Obstetricians and Gynecologists recommends that pregnant or breast-feeding women — and women considering pregnancy — should be screened for and discouraged from using marijuana and other substances.

The study published Monday analyzed data from women ages 18 through 44 from the annual National Survey on Drug Use and Health from 2002 through 2014. Researchers assessed data from 200,510 women of reproductive age, including 10,587 pregnant women.

Among pregnant women, the prevalence of the use of marijuana in the past month increased 62 per cent from 2002 through 2014. The use was most prevalent among women ages 18 to 25, indicating that younger women are at greater risk. Recent use among nonpregnant women also increased, from about 6 per cent in 2002 to 9 per cent in 2014, researchers reported.

Marijuana is the most widely used illegal drug during pregnancy, Volkow wrote, and a study from Hawaii suggested that pregnant women with severe nausea are more likely to use marijuana than other pregnant women.

Despite medical recommendations, online forums and articles laud the pain-relieving benefits of marijuana use during pregnancy, particularly to treat hyperemesis gravidarum, a severe type of morning sickness consisting of persistent nausea and vomiting that can lead to dehydration, weight loss and electrolyte imbalances.

To date, 29 states and Washington, D.C., have passed laws legalizing medical marijuana. Nausea is a medically approved reason to take marijuana in all of the states where medical use is legal. Although no states specifically list pregnancy-related conditions as an indication for the drug, none of them prohibit or include warnings about the possible harms of marijuana, Volkow wrote.

The numbers of pregnant women using marijuana — less than 4 per cent, according to the study — are not high. Nevertheless, the authors of the study wrote, “the increases over time and potential adverse consequences of prenatal marijuana exposure suggest further monitoring and research are warranted.”